Communication Networks NETW 501 Lecture 3 Layered Architecture

Communication Networks NETW 501 Lecture 3 Layered Architecture of Communication Networks: TCP/IP Model Course Instructor: Dr. -Ing. Maggie Mashaly maggie. ezzat@guc. edu. eg C 3. 220 1

Important concepts to learn about • What is a Packet? • Packet Transmission time • Packet Propagation time • Circuit Switching • Packet Switching • Virtual Circuit Packet Switching 2

What is a Packet? 1011… 00110001… 10011010… 00111000… 0101… • A packet is a small block of data bits. • Packets are sent individually • All the bits in a given packet are treated as single entity 3

Why Packets? Packets allow for a more convenient transmission process by providing the following: 1. Coordination – Helps transmitter and receiver determine which section of data has been received correctly 2. Low overhead due to Retransmission – When bit error occurs, retransmission of the erroneous packet only is needed 4

Headers and Trailers • For a layer N+1, layer N does not understand the message content • So layer N adds bits (Headers and Trailers) that represents its language (i. e. , protocol) • The header and trailer for each layer may represent: ØMessages to help the receiver side understand what to do ØParameters to help the receiver side perform its service correctly 5

Headers = Overhead • We want to send L bits • We actually send Application Data HT bits TD+HD+HN+HT+L bits TH Application Data HN bits • Header and/or trailer added NH by layer N is independent of the packet length from layer HD bits N+1 DH TH: Transport Header NH: Network Header DH: Data link Header DT: data link Trailer Transport Data TD bits Network Data DT Data Link Data TD+ HD+HN+HT+L bits 6

Throughput Efficiency For an Error Free Channel Throughput Efficiency reflects the percentage of Øinformation bits to Øthe total number of bits required for the information packet to arrive safely at the destination 7

Packet Transmission Time ØIt is the time required to put all packet bits over the physical channel ØIt is dependent on the modem transmission rate ØExample • If Transmission Rate is 1 Mbps and Packet Size is 8000 bits then • Transmission Time is: Tt = 8000/1*106=8 ms 8

Packet Propagation Time ØIt is the time it takes the electrical signals to travel from one machine to the other ØIt is dependent on the speed of light and the distance between the two machines ØExample • Assume the speed of light over cable is 2. 3*108 m/s and the distance between two machines is 1000 Km, then • Propagation Time is: Tp = 1000*103/2. 3*108=4. 345 ms 9

Example Physical Channel Machine A Machine B Tp=4. 345 ms Tt=8 ms Time 10

Circuit Switching ØThe concept of circuit-switching comes from old telephone technology ØFor each connection, physical switches are set in the telephone network to create a physical “circuit” ØAn end-to-end path MUST be set up before any data can be sent 11

Phases of Circuit Switching 1. Circuit Establishment – A circuit is established between two end users with the help of intermediate switches 2. Information Transfer – Information is transmitted using the circuit established in the set-up phase 3. Circuit Disconnect – Circuit is terminated – Dedicated resources are released 12

Circuit Switching - Example 1. Circuit Establishment 3. Information Disconnect 2. Transfer Switch C Switch D Switch A Switch B Physical copper connection set up when call is made Switching offices 13

Time Diagram (Circuit Switching) Switch A Switch C Switch D Tp Circuit Establishment Data Transfer Switch B 5 Tp Tt Circuit Disconnect Time Time 14

Circuit Switching: Pros and Cons ØSwitches are set up at the beginning of the connection (call) and maintained throughout the connection ØNetwork resources are reserved and dedicated from sender to receiver ØPropagation time is the only source of delay during data transfer ØNot a very efficient strategy! – A connection “holds” a physical line even during “silence” periods (when there is nothing to transmit) 15

Packet Switching Networks ØNo physical connection is established in advance between the sender and receiver ØEach packet is sent independently ØEach packet is stored in a router, processed and then forwarded to the next hop – STORE AND FORWARD Operation 16

Packet Switching Networks - Example Router E Router C Router A Router Congested Router D Router B Router F 17

Time Diagram (Packet Switching) Router A Tt 1 2 Router B Router D Tp 1 2 TPr 1 2 Data Transfer 1 2 Time TPr Processing Time TPr 1 2 Time Router C Time 1 2 TPr 1 1 2 2 Time 18

Circuit Switching vs. Packet Switching Circuit switching • Must set up a connection (initial delay). • Resources are dedicated – Therefore they may be used inefficiently! • Performance is predictable as resources are reserved. • • Packet switching Very small set-up delay. Efficient shared use of resources. Possible congestion and consequent packet dropping Performance is unpredictable and is a function of current traffic conditions. 19

Virtual Circuit Packet Switching ØCompromise between circuit switching and packet switching - Combines advantages of both ØPreplanned route established before any packets sent ØConnection Establishment Phase is required ØEach packet contains a Virtual Circuit Identifier instead of destination address ØNo routing decisions required for each packet ØConnection release is required after data transfer phase is completed 20

Virtual Circuit Packet Switching - Example 3. Circuit Disconnect 1. 2. Connection Information Establishment Transfer Router E Router C Router A Router D Router B Router F 21

Time Diagram (Virtual Circuit Packet Switching) Router A Router B Router C Router D Connection Establishment 1 1 2 21 TPr 2 Data Transfer 1 2 1 TPr 2 2 Time Connection Release Time 1 22

Virtual Circuit vs. Packet Switching ØVirtual Circuits – No packet reordering occurs because all packets follow the same reserved path – Packets are forwarded more quickly › Less processing time because no routing decisions are necessary – Requires connection set-up and release times – If an intermediate router is down a new connection establishment must be performed ØPacket Switching – No Set/Release Phases – More flexible › Can recover from down time of intermediate routers by selecting alternate routes without the need for any new connection establishment 23

References • NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy • “Communication Networks 2 nd Edition”, A. Leon-Garcia and I. Widjaja, Mc. Graw Hill, 2013 • “Computer Networks 4 th Edition”, A. S. Tanenbaum, Pearson International 24